1. Field of the Invention
The present invention relates to a control device for in-wheel transmissions, each of which comprises a stepped transmission that transmits power from each electric motor to each drive wheel of an electric vehicle.
2. Description of the Related Art
Electric vehicles are previously known in which an electric motor is individually provided for each drive wheel, and the power from each electric motor is transmitted to each drive wheel through each of in-wheel transmissions comprising a continuously variable transmission (see Laid-open Japanese Patent Application No.H10-89439).
Also, because the cost of employing a continuously variable transmission is high, consideration has been given to constructing the in-wheel transmissions as stepped transmissions.
Because the speed range of an electric motor is wide, in the case where the in-wheel transmission is constructed of a stepped transmission, if the number of transmission steps is two, this is functionally sufficient, but, in this case, considerable shock is experienced on gear-change. A multi-stepped transmission, i.e., a three or four stepped transmission is therefore recommended, but this gives rise to problems regarding increased weight and size.
In view of this aspect, an object of the present invention is to provide a control device for an in-wheel transmissions such as to alleviate shock but without increasing the number of steps of an in-wheel transmission comprising a stepped transmission.
In order to attain the above object, according to the present invention, a control device for in-wheel transmissions ,each of which comprises a stepped transmission that transmits power from each electric motor to each drive wheel of an electric vehicle, comprises control means whereby gear-change of the in-wheel transmissions for the drive wheels is effected with a time difference at each drive wheel.
When gear-change of the in-wheel transmissions is effected, the vehicle acceleration changes with the change of torque produced by the torque characteristic of the electric motors, so, if gear-change of the in-wheel transmissions of all the drive wheels is effected simultaneously, the amount of change of acceleration of the vehicle as a whole is the total of the acceleration changes of all the drive wheels, and therefore a considerable shock is produced. However, according to the present invention, the change in acceleration of the drive wheels produced by the gear-changes of the in-wheel transmissions is dispersed in regard to time, so the shock is alleviated.
However, in a four-wheel drive electric vehicle in which all four front and rear and left and right vehicle wheels are drive wheels, if gear-change is effected with a time difference with respect to all of the in-wheel transmissions for the four vehicle wheels, the resultant of the drive force of the vehicle wheel that is being subjected to gear-change and the drive force of the other vehicle wheel on the same side, left or right, falls below the total drive force of the two vehicle wheels on the opposite left/right side due to the drop in the drive force of the vehicle wheel that is being subjected to gear-change. This causes a yaw moment to act on the vehicle. In this case, if the output of the electric motor for the other vehicle wheel on the same side, left or right, as the vehicle wheel that is being subjected to gear-change is increased when gear-change of each of the in-wheel transmissions of the vehicle wheels is being conducted, the drop in the drive force of the vehicle wheel that is being subjected to gear-change is canceled by the increase in the drive force of the other vehicle wheel on the same side, so action of a yaw moment can be prevented.
Also, in the case of a four-wheel drive electric vehicle, even if gear-change of the in-wheel transmissions is conducted simultaneously for two vehicle wheels in each case, with a time difference, shock is alleviated compared with the case where gear-change is conducted simultaneously for the four wheels. Thus, if gear-change of the in-wheel transmissions is conducted simultaneously for one vehicle wheel on each of the left and right sides, making a total of two vehicle wheels, and gear-change of the in-wheel transmissions is conducted simultaneously for the remaining two vehicle wheels with a time difference thereafter, the yaw moment can be prevented.